docs: add missing external-services guide, fix HTML support status

[Frankie-Xu]

Summary

• Add docs/external-services.md, the dependency matrix already linked from README Quick Start and Additional Guides.
• Move .html / .htm from Coming Soon to Supported, matching SUPPORTED_EXTENSIONS and the worker HTML parser.
• Docs-only. No API, worker, deployment, or migration impact.

Closes #333

Verification

• Extracted env keys from apps/api/.env.example and apps/worker/.env.example (including commented KEY= lines). Cross-checked every documented key against pydantic config / usage in packages/shared-python/shared/core/config/ and related modules. No invented keys. No real credentials or private endpoints.
• rg on README docs/ relative links: all targets exist (docs/external-services.md, docs/adr/README.md, docs/adr/0004-anonymous-self-hosted-telemetry.md, and the three docs/assets/ images).
• UV_PYTHON=3.13 make lint — All checks passed.
• Not tested: API, worker, or local-dev runtime (no code or env-default changes).

Deployment Notes

• No new or changed environment variables.
• No database migrations, queue changes, storage changes, or release-order requirements.
• Backwards compatible; docs-only rollback is reverting this PR.

Checklist

• Tests were added or updated when behavior changed — N/A (docs-only)
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy — N/A
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change — README now lists HTML as supported; no API contract change